Torque meters, sometimes referred to as power meters, have long been used to determine mechanical loads on rotating shafts in turbo machinery. By determining torque, the operator of the equipment can determine if the equipment is performing within its material strength limits, and, if combined with the rotating speed of the system, the energy the system is consuming or delivering per unit of time (power) can be determined. Power (in Watts) is defined as the multiplication of torque (in N*m) and rotational speed (in rad/s). Measuring the speed of a shaft is straightforward with the use of standard speed measurement devices (tachometers) that measure rotational speed in Hertz (rad/s) or revolutions per minute (rpm). Different technologies are widely available to measure rotating speed, and it is common practice to measure the speed of turbo machinery as a means to monitor the “health” of the equipment for product performance and maintenance purposes. With known torque and speed variables, the power captured or consumed by turbo machinery can be determined. This enables the continuous monitoring of the efficiency of the equipment, where efficiency is defined as the ratio of power (or energy) output over power (or energy) input. This type of health monitoring can provide turbo machinery operators the ability to continuously monitor the performance of the equipment based on real-time data (rather than predictive/error-prone methods) and take proactive actions to recondition equipment to address performance degradation and/or avoid costly or fatal failures. Despite the benefits, measuring the torque of turbo machinery in field installations has been an elusive goal. The existing techniques used to measure torque in rotating equipment are either too costly, or heavy, or impractical, or are not robust enough to withstand the challenges in the field.
Several methodologies exist today to measure torque. Some relate to static torque measurements that apply to non-rotating components, and some refer to dynamic torque measurements that apply to rotating equipment, which are of greater concern in turbo machinery and, therefore, the focus of the method of the present invention. The techniques to measure dynamic torque include the following: (a) installing strain gauges on the surface of the shaft in a Wheatstone bridge circuit configuration; (b) using a torque transducer that is directly coupled to the shaft; or (c) using optical torque measurement techniques. These techniques are described in the following paragraphs.
Strain Gauges:
Strain gauges are usually placed on the surface of the shaft and, as the shaft deforms due to the applied torque (sometimes referred to as mechanical load), the strain gauges also deform, which changes the electrical resistance of the strain gauge, which, in turn, causes a change in voltage that is proportional to the strain or mechanical deformation in the surface of the shaft. Strain (measured in m) is directly proportional to the mechanical stress (measured in N/m2) if the material is within its elastic limit according to Hooke's law. Then, if the shaft dimensions are known, the amount of torque applied to the shaft can be determined. This technique assumes that the shaft material properties (including type of material, modulus of elasticity, etc.) are known and that the relationship between torque and stress is also known. In most cases, the material behaves elastically, but, as the material moves towards non-elastic behavior, the use of these techniques becomes subject to larger measurement errors. If the material is made from a non-elastic type, such as plastic, then the relationship between torque and stress/strain becomes less predictable.
Moreover, to install the strain gauges on a shaft, several electric connectors (wires) are needed to connect the strain gauge Wheatstone bridge circuit to a data acquisition system. These types of arrangements are cumbersome, not very robust and can be easily broken during use, especially if the shaft is rotating. Therefore, strain gauge measurement methods are typically limited to static load laboratory tests, where a non-rotating shaft of known dimensions is subjected to a known or unknown torque load, and the amount of stress is then calculated based on the deformation (strain) read from the strain gauges. Furthermore, strain gauge measurements typically require several strain gauges to be installed in order to have redundant readings, since strain gauges are also susceptible to temperature changes due to thermal expansion in the base material, along with vibrations and other noise. This redundancy adds to the complexity of the strain gauge installation and limits their use to non-commercial/field installations.
Torque Transducer.
Dynamic torque can be measured by directly coupling a torque transducer on a rotating shaft. Transducers typically include a load cell, which is also built using strain gauges. The main issue with these torque meters is that they are costly and sensitive pieces of equipment susceptive to failure when exposed to excess loads, vibration or thermal expansion. These factors limit their use to small scale, lab-type applications, with limited deployment in the field.
Optical Torque Measurement Devices.
Optical torque measurement devices include those devices where a shaft insert (torque meter) of predetermined characteristics is placed inline with the main shaft of the equipment subjected to dynamic torque. As the torque meter suffers angular deflections, these deflections can be measured optically/electronically by using lasers that indicate the amount of angular deformation; this angular deformation is, in turn, directly proportional to the applied torque.
In summary, current static and dynamic torque measurement devices and methods rely on some form of measurement of the mechanical deformation on either the actual main shaft of the equipment, as in the case of strain gauges, or the mechanical deflection of an element attached directly to the main shaft (transducers). These methods have proven to be accurate in controlled environments, but they have proven to be too costly, fragile or heavy to be deployed in actual commercial installations. These limitations inspired this inventor to develop a new and robust method to measure dynamic torque in commercial/field installations of turbo machinery, one which enables measuring real-time efficiency and improved health monitoring.
The present invention is a method that calculates the dynamic torque by applying dynamic principles, measuring two main state variables (speed and time) on a real-time basis, in combination with the inertia of the system, and torque associated with non-conservative forces which was measured previously, during the natural decay of the system in similar operating conditions. The estimated torque the method produces can be used to monitor or control the speed of the system to keep the equipment within its structural limits of performance, to extend the life of the equipment, and to optimize the system to maximize energy efficiency on a real-time basis. Therefore, this method not only overcomes the limitations described earlier of current technologies, which base their torque measurements on material deflections, but it also enables the optimized use of turbo machinery and rotating process equipment on a real-time basis. Finally, because of its simplicity, the proposed invention can be used in new or existing (as a retrofit) turbo machinery and rotating equipment.
No prior art was found that used rotational speed to calculate torque in both steady state (constant speed) and unsteady state (varying speed) conditions, based on the use of Newton equations of motion (as shown in equation 1, infra) and by incorporating values of the non-conservative forces (or conservative torques) calculated during the natural decay of the system and stored for later use, to calculate the torque of the system on a real-time basis in similar speed and operating conditions. The method proposed can be used with a large variety of rotating equipment and turbo machinery.
The prior art involving a method that use rotational speed, directly or indirectly, to estimate or set a target torque includes U.S. Pat. No. 9,014,861 to Attia, in which the invention relates to a method to control noise in wind turbines, where wind speed readings lead to a target rotational speed and, consequently, a target torque and target pitch of the wind turbine blades. However Attia only applies to wind turbines, and it cannot be applied generally to rotating machinery other than a wind turbine, whereas the method of the present invention overcomes these limitations. Further, Attia does not use the Newton equations of motion or data taken from the natural decay of the system to calculate torque on a real-time basis. The method of the present invention does use Newton's equations of motion, real-time rotational speed measurements and data taken from the natural decay of the system to calculate the torque that is being applied to a rotating machine.
Another reference, U.S. Pat. No. 7,352,075 to Willey, et al., teaches a method to control the rotating speed and rotor torque based on controlling the torque in the generator of a wind turbine. Willey shows a graphical relationship between wind speed, rotor rpm and power output from the wind turbine, but it fails to teach specifically how rotational speed can be used to derive the applied torque; it does show how the wind speed and rotational speed can be used to set a target torque in the electrical generator and, therefore, determine the required blade pitch. Like Attia, the Willey reference is specific to wind turbines, and it does not apply generally to rotating equipment, as does the present invention.